When manufacturing cellulose by sulphate cooking method, wood is cooked in an alkaline liquor containing sodium hydroxide and sodium sulphide. The lignin of the wood is separated from actual cellulose fibres. The cooking liquor and cellulose pulp are separated, and the pulp is then washed. The fibres from the pulp are then bleached and/or finished otherwise. The residual liquor, referred to as "black liquor", is further processed for recovery and reuse.
Black liquor contains many chemicals which can be recycled back to the cooking process following retreatment to reduce the amount of new chemicals required for the process. Black liquor also contains organic materials which can be burned for energy recovery provided that the liquor is sufficiently concentrated.
Black liquor is usually concentrated by utilizing the vapor produced at various stages of the process and the heat energy contained therein to evaporate water from the black liquor. The concentrated black liquor is then fed into a soda recovery boiler where it is burned and forms a smelt. Green liquor is prepared from the smelt from the soda recovery boiler and changed into white liquor, i.e. alkaline cooking liquor, by adding lime.
Sulphur enters a sulphate cellulose plant in the form of sulphuric acid from the manufacture of bleaching chemicals and for splitting tall oil in a tall oil cooking plant. This sulphur in a variety of resulting forms enters the pulp cooking process and must be addressed therein. Particularly troubling are the sulphurous gases generated during the process. Examples of sulphur gas discharge points include the cellulose cooking stage, the evaporation plant where black liquor is concentrated, and in the soda recovery boiler when black liquor is burned.
Excess sulphur poses a number of problems in a sulphate cooking process. Sulphur deposits foul equipment and reaction contact surfaces. Sulphur is also an odor nuisance and is considered to be a harmful emission to the environment. A reduction in sulphur gas emissions (e.g., by increasing the dry content of the liquor or by collecting all odor gases of the plant together to be destroyed by burning) causes an increase in sulphur in the liquor cycle. It would be desirable to have a process that would remove a desired amount of sulphur from the liquor cycle in order that the sulphur content of the cycle could be maintained as desired.
From the Finnish Published Specification 85515 (U.S. Pat. No. 5,277,759), for instance, sulphur is removed as a gas from black liquor by heating the black liquor before the last concentration stage to a temperature higher than the cooking temperature and maintaining that temperature for at least 20 minutes. Thereafter, the black liquor is permitted to expand and vapors containing gaseous sulphur compounds are released from the black liquor. The sulphurous gases are destroyed by combustion. The black liquor is further concentrated by further evaporation. Though obviously functional, the equipment required for the process is very large, is expensive, and has a very poor energy economy.
On the other hand, U.S. Pat. No. 2,711,430 teaches that gaseous sulphur compounds can be separated with water vapor from the black liquor by heating the black liquor to a temperature higher than the cooking temperature, maintaining that temperature for a sufficient time, and allowing the black liquor to expand. The amount of the sulphur separated is varied based on the temperature and especially on the delay time.
It would be desirable to have a process that would allow excess sulphur to be removed from the liquor cycle while preserving energy values where possible.